Process for making multiple layer polymeric films

ABSTRACT

Novel films, and processes for making them, are provided. The films are characterized by the inclusion of a layer of vinylidene chloride copolymer between at least two other layers which contain ethylene vinyl acetate, and optionally linear low density polyethylene. The films may be unoriented or oriented. Oriented films may optionally be cross-linked. The novel process of making the cross-linked films includes the step of cross-linking the film after assembly of the vinylidene chloride copolymer layer into the structure.

BACKGROUND OF THE INVENTION

Heat shrinkable polymer films have gained substantial acceptance forsuch uses as the packaging of meats. This description will discuss theusage of films for packaging meat; it being understood that these filmsare also suitable for packaging other products. Some of the filmsembodying this invention are normally used as heat shrinkable bagssupplied to a meat packer with one open end, to be closed and sealedafter insertion of the meat. After the product is inserted, air isnormally evacuated, the open end of the bag is closed, such as by heatsealing or applying a metal clip, and finally heat is applied, such asby hot water or hot air, to initiate shrinkage about the meat product.

In subsequent processing of the meat, the bag may be opened and the meatremoved for further cutting of the meat into user portions, for retailsale, for example, or for institutional use.

Successful shrink bags must satisfy a multiplicity of requirementsimposed by both the bag producer and the bag user. Of primary importanceto the bag user is the capability of the bag to survive physicallyintact the process of being filled, evacuated, sealed closed, and heatshrunk. The bag must also be strong enough to survive the materialhandling involved in moving the contained product along the distributionsystem to the next processor, or to the user. Thus, the bag mustphysically protect the product.

It is also highly desirable to the bag user that the bag serve as abarrier to infusion of gaseous materials from the surroundingenvironment. Of particular importance is provision of an effectivebarrier to infusion of oxygen, since oxygen is well known to causespoilage of food type products.

The bag producer requires a product which can be produced competitively,while meeting the performance requirements of the user. Thus the bagmaterial should be relatively inexpensive to purchase, should be readilyextrudable, and susceptible to orientation, with sufficient leeway inprocess parameters as to allow for efficient film production. Theprocess should also be susceptible to extended production operations.

The orientation temperature should be a temperature which iseconomically achieved by the producer, and which provides for use ofeconomical shrink processes by the bag user.

During fabrication and use of the film, it must be tough enough towithstand the various high temperature operations to which it issubjected, including heat sealing, and in some cases shrinking. Thus,its strength at high temperature, hereinafter referred to as hotstrength, is an important consideration.

Conventional shrink bags have generally been constructed with ethylenevinyl acetate copolymers (EVA). In some cases the bags contain a layerof a vinylidene chloride-vinyl chloride copolymer (VDC-VC) to serve asan oxygen barrier. Ethylene vinyl alcohol copolymer (EVOH) is also knownas an oxygen barrier material.

Notwithstanding the several shrink films which are available, and theadvantages of shrink packaging, shrink packaging is not without itsdifficulties, many of which are attributable to limitations in the film.As will be appreciated, the processes of stretching the film, and latershrinking it, expose the film to rather severe conditions, due to thenature of the operations.

It is especially important to appreciate that the film is particularlyvulnerable to failure at conditions of operation, due to the hightemperatures to which it is exposed in the orientation and shrinkingprocesses. The film must be susceptible to orientation withoutdistortion or separation of the multiple layers which are normallypresent in films of this nature. The film must be strong enough, at theorientation temperature to withstand the stresses of stretching withoutthe creation of holes, tears, or non-uniform zones of stretching.

In the case of tubularly oriented films, the film must be capable ofsupporting the stretching bubble during the orientation process.Finally, each of the layers of the film should be susceptible toorientation without fracture, separation, or creation of holes in thelayer.

In packaging use, the film must respond to heat rapidly enough in theshrinking process for commercial practicality and yet must not exhibitsuch a level of shrink energy as would cause the film to pull apart ordelaminate during shrinkage, under its own internal forces. Moreover,the shrink related problems are increased when the contained product,such as a cut of meat includes protrusions such as bones, and/orsignificant cavities in its surface.

Particularly in the case of cavities in the product, such as around theinterior of the rib section in a cut of meat, the redistribution of anarea of the film adjacent the cavity places an extra strain on theability of the film to conform to the product in the shrinking processwhile maintaining film continuity.

Another area where film packages are known to be susceptible to failureis at any area where portions of the film are sealed to each other by aheat seal. In the formation of a heat seal, at least portions of thefilm are heated to a temperature where they are soft enough to flow andbe melt merged when simultaneously subjected to pressure. It isdesirable to be able to form heat seals in a film over a range oftemperatures and pressures so that commerical processes can fluctuatewithin the normal operating parameters. Whatever the acceptable range ofconditions of formation of heat seals, it is critical that the sealshave adequate strength to hold the package closed, and prevent leakageinto or out of the package until it is intentionally opened. Thus thestrength of heat seals is also one of the important measures of thevalue of films which are used in applications where heat seals areformed.

The common factor in all these situations is that the film is heated toa high temperature, at which it may be softened, and an operation isperformed, usually by deformation such as stretching, shrinking, andsoftening and merging to form a heat seal. While the film needs to besufficiently deformable to perform a desired function, it need also havesufficient hot strength to not become so soft that it flowsuncontrollably and assumes undesired shapes, such as by melting,developing holes, and the like.

It is generally known that cross-linking of polymer films improves theirtoughness and hot strength. It is known as a process to cross-link onelayer of a multiple layer film containing a VDC-VC copolymer. Thiscross-linking of a single layer of a multiple layer film consists of aplurality of steps. For example, first the layer to be cross-linked isformed. Second, the formed layer is cross-linked. Third, additionallayers are added to the cross-linked layer, as by extrusion coating, toform a multiple layer film. Finally, the multiple layer film is heatedto orientation temperature and oriented. While this process may producea functional film, it would be desirable to invent a process which mightbe less complex, require fewer steps, perhaps improve the inter-layeradhesion, and perhaps be more economical.

As regards the above described known process, it is seen that only oneof the layers is cross-linked. A typical film has two outer layers ofEVA and an inner layer, between the two EVA layers, of VDC-VC copolymer.One of the EVA layers is cross-linked and the other is not. With aplurality of processing steps required to form a film by theabove-iterated known process, it is seen that processing economics mightbe attained by a different process, particularly if the number ofprocessing steps can be reduced.

It is an object of this invention to provide improved film structuresfor use in packaging, especially for use in polymeric bags; and processfor making film structures and packages. It is a special object to makefilms having improved properties for packaging uses, and to make them byprocesses which are competitive and economical as compared to previouslyavailable processes.

SUMMARY OF THE INVENTION

It has now been found that these and other objectives are achieved innovel films and processes of the invention. Films of the invention arerepresented by a multiple layer polymeric film having first and secondlayers whose compositions have a significant fractions of EVA. A thirdlayer of a vinylidene chloride copolymer (VDC-CP) is disposed betweenthe first and second layers.

In some embodiments, the composition of at least one of the first andsecond layers is a blend of 10% by weight to 90% by weight linear lowdensity polyethylene (LLDPE) and 90% to 10% EVA.

In more preferred embodiments, the compositions of the first and secondlayers are 20% to 40% LLDPE and 80% to 60% EVA. In these more preferredembodiments, the EVA is characterized by having 6% to 12% vinyl acetatecontent and by having a melt index of 0.2 to 0.8. The LLDPE has a meltindex of 0.5 to 1.5.

While the composition of the third layer may be any of the vinylidenechloride copolymers, preferred compositions for the third layer arevinylidene chloride-acrylate type copolymers, especially vinylidenechloride-methylacrylate (VDC-MA).

The films of the above-described layers, with their various combinationsof compositions in the layers, are highly useful as shrink filmproducts, and so it is customary, though not essential, that films ofthe invention be molecularly oriented. And while the films of theinvention may be oriented without cross-linking of any of the layers,cross-linking can impart desired improvements in the shrinking, heatsealing, and hot strength properties, and perhaps interlayer adhesion.Thus the most preferred films of the invention are irradiated in orderto induce cross-linking in the film. Preferred levels of irradiation areof the order of 1.5 to 10 megarads.

In the most preferred films of the invention, the composition of each ofthe first and second layers is again made from a major fraction of EVAcopolymer. A third layer is made from VDC-MA copolymer, and is betweenthe first and second layers. By the time the fabrication of the film hasbeen completed, all of the first, second and third layers have beensubjected to electron beam irradiation in an amount of at least 1.5megarads.

In another family of embodiments of the preferred oriented films of theinvention, the first and second layers each have two opposing surfacesand have essentially the same composition, each as the other; the firstand second layers being defined as a first pair of layers. The thirdbarrier layer of VDC-CP is between, and in surface-to-surface contactwith the first and second layers. Fourth and fifth layers are adhered tothe first and second layers on the respective surfaces opposite thethird layer. The fourth and fifth layers have essentially the samecomposition, each as the other, and are defined as a second pair oflayers. In the combined composition of the first and second pairs oflayers, the composition of at least one of the pairs is at least 50% ofan EVA component, the remainder of that one pair being an LLDPE. Also,the composition of at least one of the pairs is at least 10% of an LLDPEcomponent, with the remainder of that one pair being EVA. Therequirement for the components of the at least 50% EVA and the at least10% LLDPE may be met by one of the pairs having both components or byeach of the pairs having one of the components.

In one more preferred group of films of this family of embodiments, thefirst pair of layers is 70% to 100% EVA and the second pair of layers is10% to 90% LLDPE.

In another more preferred group of films of this family of embodiments,the first pair of layers is 100% EVA and the second pair of layers is50% to 90% LLDPE.

In a group of films in this family of embodiments where the roles of thefirst and second pairs are somewhat reversed from those just described,the first pair of layers is 50% to 100% LLDPE and the second pair oflayers is 50% to 100% EVA. Indeed, in one preferred version of thisgroup, the first pair of layers is 90% to 100% LLDPE and the second pairof layers is 90% to 100% EVA.

The films of the above-described family of oriented films having atleast 5 layers may be successfully fabricated and used without thecross-linking of all of the several polymer layers. However, as with thepreviously described families of films, which may have less than fivelayers, the hot strength and heat seal properties of these films may beenhanced by subjecting them to cross-linking processes; and so it isespecially desirable that the first, second, and third layers, of theembodiments having at least 5 layers, be cross-linked by exposure to atleast 1.5 megarads of electron beam irradiation.

The invention includes special and novel processes for making thecross-linked films of the invention. The first step is forming amultiple layer film having in it the layers to be exposed to theelectron beam irradiation, including a layer of VDC-CP. The second stepis heating the multiple layer film to molecular orientation temperatureand molecularly orienting it. The third step is subjecting the multiplelayer film to electron beam irradiation in an amount of at least 1.5megarads, preferrably 2 to 5 megarads. Optionally, the film may be heatset.

The films of the invention are highly satisfactory for many purposes,including making flexible bags and the like for packaging use. Certainones of the oriented films are highly desirable for use in making shrinkbags, wherein the bag is caused to shrink about the contained product,by the application of heat to the bag to activate its shrink properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a bag made according to the invention.

FIG. 2 is a cross-section of a bag of FIG. 1, the bag having been madefrom a 3 layer film structure of the invention, and taken at 2--2 ofFIG. 1.

FIG. 3 is a cross-section as in FIG. 2, but showing a bag made from a 5layer film structure of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a bag 10 made according to the invention. The empty bagshown is illustrative of bags of the invention. In the most preferredform of the invention, the bag is made from a molecularly oriented tubewhich has been subjected to radiation cross-linking, and except whereotherwise pointed out, the remainder of this description will describefilms and packages which have been molecularly oriented by the time ofthe completion of the manufacturing process, and which are useful forpackaging foods. In FIG. 1, then, the molecularly oriented tube has oneend closed by a heat seal 12 across the one end of the tube. The otherend of the bag is open for insertion of product, and is normally closedand sealed after the product is put into the bag.

The cross-section of the bag in FIG. 2 shows a typical structure wherethe bag is made from a three-layer plastic film. Layer 14 is a barrierlayer made from vinylidene chloride copolymer. Layer 16 is the heat seallayer. Layer 18 is the outer bag layer and serves a primary function ofprotecting the package and its product from physical abuse. In the formof the invention using a three-layer film as in FIG. 2, layer 18 is ablend of 10 weight percent to 100 weight percent of an EVA and 90 weightpercent to 0 weight percent LLDPE. Layer 16 is also 10% to 100% EVA and90% to 0% LLDPE. The inclusion of LLDPE is particularly desirable wherethe film is to be used as a shrink film, where it is heat shrunk aboutan enclosed product. While LLDPE is generally useful for enhancing thetoughness of the film, films having LLDPE blended into at least one ofthe outer layers of a three-layer film exhibit particularly significantreduction in package failure rates from shrink holes, when used inshrink packaging. While smaller amounts of LLDPE do provide someimprovement, generally at least 10% LLDPE is used where significantbenefits are desired. Thus 10% LLDPE in at least one of layers 16 and 18is preferred. Films having 20% to 40% LLDPE show marked improvement, sothis level is especially preferred.

LLDPE polymers preferred for use in layers 16 and 18 which are blendcompositions are those having a melt index (MI) of up to about 6.Especially preferred polymers have an MI of 0.5 to 1.5. Among the mostpreferred polymers are 2050 and 2056 from Dow Chemical Company.

As used herein, the term melt index refers to the physcial propertydetermination described in ASTM-D1238.

The ratio of LLDPE in the blends which use it is selected to provide thebest balance of properties which maximizes desirable benefits of each ofthe elements of the blend for the anticipated use of each specific film.While initial improvements in the film, compared to films havingstraight EVA in layers 16 and 18, are seen in films having as little as10% LLDPE in layers 16 and 18, films having 20% to 40% LLDPE show markedimprovements.

EVA's having lower VA content tend to yield EVA layers having better hotstrength. EVA's having higher VA content tend to yield EVA layers havingincreased adhesion to the vinylidene chloride copolymer layer. EVA'shaving virtually any amount of VA will have better adhesion to thevinylidene chloride copolymer layer than an ethylene homopolymer.However, good interlayer adhesion is considered desirable in theinvention, and thus steps are usually taken to enhance adhesion where nounacceptable negative effect is encountered. Thus, higher VA contents,in the range of 6% to 12% vinyl acetate are preferred. Melt index ofless than 1 is preferred. While blend amounts are shown herein in weightpercent, VA contents are mole percent. Especially preferred EVA's haveVA content of 7% to 9% and melt index of 0.2 to 0.8. Blends of EVA's tomake up the EVA component of layers 16 and 18 are acceptable and, insome cases, facilitate the orientation process.

The composition of layer 14 is a vinylidene chloride copolymer.Especially preferred is a vinylidene chloride-methylacrylate copolymer.Where methylacrylate copolymer is used, the methylacrylate component ofthe copolymer is preferably between 3 mole percent and 20 mole percent.Highly preferred copolymers have 6 to 12 mole percent methylacrylate.Additives typically used with vinylidene chloride copolymers may be usedin conventional amounts. Exemplary of such additives are EVA,dibutylsebacate, magnesium oxide, stearamide, and epoxidized soybeanoil.

The overall thickness of the films of this invention is nominally thesame as the thickness of conventional films. Films are generally about2.0 mils thick with a normal range of about 1.5 to about 3.0 mils. Filmsthinner than about 1.5 mils tend to be too weak to perform all requiredfunctions. Films thicker than about 3.0 mils are economically lesscompetitive, although films up to about 20 mils are functional.

The thickness of each layer of the shrink films of this invention ispreferably essentially the same as the thickness of the same layer inconventional shrink films. By way of example, in a typical film used tomake a bag of FIGS. 1 and 2, the overall film thickness is 2.25 mils.Layer 14 is 0.3 mil. Layer 16 is 1.45 mils. Layer 18 is 0.5 mil. Layerthicknesses may be conveniently adjusted for any particular film. Aminimum of 1.0 mil is desired for layer 16 where it is to be used forheat sealing purposes.

The compositions of the various layers are discussed herein as thoughthe composition of any one layer is constant with time. It isconsidered, however, that the compositions, and particularly themolecular structures of the various polymers, are changed bycross-linking affect of the irradiation. Thus the description of thepolymers, while general, should be taken to include subsequent formsthereof after irradiation.

THE PROCESS

The process of making any given film does, of course, depend on thespecific composition and structure of the film, whether it is to beoriented, and whether it is to be cross-linked.

Films which are neither oriented nor cross-linked can be made by any ofthe conventional processes for forming multiple layer films. Suchprocesses include extrusion, coextrusion, extrusion coating, extrusionlamination, adhesive lamination and the like, and combinations ofprocesses. The specific process or processes for making a given filmwhich is neither oriented nor cross-linked can be selected with averageskill, once the desired structure and compositions have been determined.

Films which are oriented, but not cross-linked, can also be made byconventional processes, in combination, for forming multiple layerfilms. A preferred process includes the steps of coextrusion of thelayers to be oriented, followed by orientation in one of theconventional processes such as blown tubular orientation or stretchorientation in the form of a continuous sheet; both being molecularorientation processes.

Films which are oriented and are cross-linked are made by a novelcombination of process steps. The first step is the formation of amultiple layer film. The first step, of formation of the multiple layerfilm, is usually most easily accomplished by coextrusion of the desiredlayers of which the vinylidene chloride copolymer layer is one. Otherformation processes are acceptable so long as the resulting orientedfilm at the conclusion of fabrication processing is a unitary structure.

The second step is orienting the multiple layer film. This isaccomplished by heating the film to a temperature appropriate tomolecular orientation and moleculary orienting it. The film may then beoptionally heat set by holding it at an elevated temperature while itsdimensions are maintained. The orientation step is preferentiallycarried out in line with the first step, which is the film formationstep of the process.

The third step is subjecting the formed and oriented multiple layerfilm, including the vinylidene chloride copolymer layer, to electronbeam irradiation.

The amount of electron beam irradiation is adjusted, depending on themake-up of the specific film to be treated and the end use requirement.While virtually any amount of irradiation will induce somecross-linking, a minimum level of at least 1.5 megarads is usuallypreferred in order to achieve desired levels of enhancement of the hotstrength of the film and to expand the range of temperatures at whichsatisfactory heat seals may be formed. While treatment up to about 50megarads can be tolerated, there is usually no need to use more than 10megarads, so this is a preferred upper level of treatment; the mostpreferred dosage being 2 to 5 megarads.

The third step of subjecting the film to electron beam irradiation isperformed only after the multiple layer film has been formed, and aftermolecular orientation, in those embodiments where the film ismolecularly oriented. It should be noted that, in the irradiation step,all of the layers in the film are exposed simultaneously to theirradiation source, such that irraditation of all the layers of the filmtakes place simultaneously.

In one embodiment of the process, the second step of orientation may beomitted and the unoriented multiple layer film may be cross-linked byirradiation treatment to produce a cross-linked, unoriented, multiplelayer film.

By the time processing of the film has been completed, the film has beenreturned to ambient temperature. Whether this occurs before or afterradiation treatment is not important to success of the process andfunctionality of the film.

EXAMPLE 1

A three layer film was coextruded. The two surface layers of the filmwere a blend of 35% LLDPE and 65% EVA. The LLDPE was DOW 2050. The 65%EVA in the composition was 25% USI NA 235 and 40% DuPont 3135X, bothpercentages being based on the overall composition of the entireEVA-LLDPE blend. The core layer, which was positioned between the twoouter layers was vinylidene chloride-methylacrylate copolymer. The thuscoextruded film was heated to orientation temperature and biaxiallyoriented at a stretch ratio of 3.5×2.5/1. After orientation, some of thefilm was treated with 4.5 megarads of electron beam irradiation, anduntreated samples were kept for comparison testing. The finished filmwas 2.3 mils thick, with 0.25 mil being the core layer.

EXAMPLE 2

A three layer film was coextruded. The two surface layers of the filmwere a blend of 30% LLDPE and 70% EVA. The LLDPE was DOW 2056. The 70%EVA in the composition was 30% USI NA 235 and 40DuPont 3135X, bothpercentages being based on the overall composition of the entireEVA-LLDPE blend. The core layer, which was positioned between the twoouter layers, was vinylidene chloride-methylacrylate copolymer. The thuscoextruded film was heated to orientation temperature and biaxiallyoriented at a stretch ratio of 3.5×2.5/1. After orientation, some of thefilm was treated with 4.0 megarads of electron beam irradiation, andsome with 8.0 megarads, and untreated samples were kept for comparisontesting. The finished film was 2.3 mils thick, with 0.3 mil being thecore layer.

EXAMPLE 3

A three layer film was coextruded as in EXAMPLE 1, except that thecomposition of the core layer was vinylidene chloridevinyl chloridecopolymer. Overall film thickness was 2.3 mils, with 0.45 mil being thethickness of the core layer. After orientation, some of the film wastreated with 1 megarad of electron beam irradiation, some with 3megarads and some with 5 megarads.

Table 1 shows properties, not otherwise shown, of the polymers citedabove and used in the examples.

                  TABLE 1                                                         ______________________________________                                        Polymer Properties                                                            Cited       Type of    Melt    Other                                          Polymer     Polymer    Index   Property                                       ______________________________________                                        NA 235      EVA        0.35    4.5% VA                                        3135X       EVA        0.35    12% VA                                         2050        LLDPE      1       --                                             2056        LLDPE      1       --                                             ______________________________________                                    

The films made in the above examples were tested for oxygenpermeability, free shrink and hot strength of heat seals. Oxygenpermeability was measured on a MOCON Oxygen Analyzer at 73° F., 100%R.H. and was not substantially affected by the radiation treatment.

In the free shrink test, square samples were cut 100 millimeters on aside and marked for identification in the with machine direction and thecross machine direction. Each sample was placed between two screens andimmersed in hot water at 200° F. for 60 seconds. The samples werewithdrawn from the water, dried, and measured in both the with machineand cross machine directions. The amount of shrinkage, in millimeterswas noted directly as the percent free shrink.

In the test for the hot strength, two strips of film, each one inchwide, were placed in face-to-face relationship, with the thickestlayers, layers 16, facing each other. These strips were joined togetheron one end using 2-sided adhesive tape. The joined ends were fastened inface-to-face relationship, to a stationary clamp. The other end of oneof the two strips was firmly clamped to a stress gauge. Approximately 2inches from the stress gauge, an impulse heat seal was formed across thewidth of the two facing test strips. The seal was formed by an impulsesealer set for 30 volts and 55 amps and a dwell time of 0.7 second.Immediately after formation of the seal, and while the film was stillhot, the stress gauge was pulled in a straight line direction away fromthe stationary end, a distance of about an inch, more or less.

As the gauge was pulled, putting tensile stress on the film, the stressinduced elongation primarily at the still-hot seal area. The seal areacontinued to elongate and in some cases it broke. The amount of pullingforce registered by the stress gauge initially increased as the gaugewas pulled, until the maximum stress was recorded. The maximum stressrecorded was reported as ounces of pulling force. This pulling force isa measure of the hot strength of the film which corrolates to thecapability of the film to withstand hot processes, including theformation of heat seals.

Table 2 shows the results of the free shrink and hot strength tests onthe examples.

                  TABLE 2                                                         ______________________________________                                        Test Results                                                                                    Free Shrink  Hot                                            Sample            MD      CMD      Strength                                   ______________________________________                                        Example 1 - No irradiation                                                                      40%     48%      3   oz                                     Example 1 - 4.5 megarads                                                                        36%     44%      22  oz                                     Example 2 - No irradiation                                                                      40%     48%      3   oz                                     Example 2 - 4 megarads                                                                          36%     43%      11  oz                                     Example 2 - 8 megarads                                                                          34%     40%      39  oz                                     Example 3 - 1 megarad                                                                           40%     48%      5   oz                                     Example 3 - 3 megarads                                                                          37%     46%      11  oz                                     Example 3 - 5 megarads                                                                          36%     44%      24  oz                                     ______________________________________                                    

As can be seen from Table 2, the free shrink was slightly decreased bythe radiation treatment, but was not changed by any amount that wouldaffect the overall utility of the film. Hot strength, on the other hand,was greatly enhanced in those films which had been subjected toradiation treatment.

A more complex form of the invention is a 5 layer polymeric structure asseen in FIG. 3. In this structure, layer 114 typically represents thebarrier layer. Layer 118 serves as the exterior, abuse resistant layer.Layer 120 is the interior, or heat seal layer. Layers 116 and 122 serveprimarily as transition layers between layer 114 and layers 118 and 120.Layers 116 and 122 may also provide, as can any of the layers, certaindesirable structural and strength benefiting properties.

In one structure, layers 116 and 122 are EVA and layers 118 and 120 areeither LLDPE or a blend of LLDPE with EVA. Layer 114 is the barrierlayer of VDC-CP, and preferably VDC-MA. In another structure, layer 114is VDC-CP, layers 116 and 122 are LLDPE and layers 118 and 120 are EVA.Likewise, both pairs of layers, wherein 116 and 122 are a first pair and118 and 120 are a second pair, may be blends of LLDPE and EVA.

Layers 114, 116, and 122 are cross-linked by exposure to at least 1.5megarads of electron beam irradiation. Alternately, all the layers ofthe 5 layer film may be simultaneously cross-linked by exposure toelectron beam irradiation.

The irradiation serves at least two significant purposes. First, itenhances the hot strength of the film. This is evidenced by expandedranges of heat sealing temperatures, and by reduced failure rates inpackages which have been heat shrunk or heat sealed. Second, the timingof the irradiation treatment being after the formation of the multiplelayer film, substantial freedom is available in selecting the processfor fabricating the multiple layer film. Thus the processes which tendto yield higher interfacial adhesion, such as coextrusion, arepreferred. Because more desirable formation processes can be used, theresulting films may have substantially improved interfacial adhesionover similar films made by less desirable processes. For example, thepreviously known film produced by the previously known process, bothbeing described briefly in the Background of the Invention herein, andwhich process uses a coating step in the film formation, was tested.Peeling was started with the aid of solvent. Once peel was started, thepeeling could be easily propagated by force of 10 to 15 grams per inchwidth. Those skilled in the art will recognize that this level of peelstrength represents a low level of interfacial adhesion. By comparison,films of EXAMPLES 1 and 3 were tested for peel strength.

In all the peel tests, samples used were strips one inch wide by about 3to 6 inches long. To start the layers in separation, one end of thestrip was wetted in a solvent of either acetone or methyl ethyl ketoneto facilitate initial layer separation. Initial layer separation wascarefully facilitated and was propagated by hand until a sufficientlength had been separated. The separated layers could then be attachedto the jaws of an Instron tensile analyzer. The layers were then pulledapart and the interfacial adhesion was recorded as the maximum grams offorce used to pull the layers apart at the interface.

In attempting to determine the comparative interfacial adhesion of filmsof the invention, it was found in all cases that the layers could not beseparated by the conventional technique of separating the layers by useof adhesive tape. In some cases, separation could not even be initiatedwith the use of solvents. In all other cases, separation could beinitiated by the use of solvent, but upon pulling as per the Instrontensile test, failure of one of the film layers was observed. This isinterpreted to mean that interlayer adhesion strength exceeds thetensile strength of at least one of the layers, and thus the layerscannot be separated in the conventional sense of peeling them apart atany one of the layer interfaces.

As evidenced by the hot strength test, the films of the invention canreadily be made into bags for packaging using heat seals. Thus it isseen that the invention provides novel film structures and bags, andnovel processes for making films having improved properties forpackaging uses; the novel processes being competitive and economical byvirtue of a reduction in the number of processing steps and by each ofthe steps being susceptible to being performed on conventional-typeequipment.

The word package as used herein is defined to include container articleswhich do not have a product therein as well as container articles whichdo have product contained therein. A package may be sealed or may havean opening therein.

Having thus described the invention, what is claimed is:
 1. A processfor making a multiple layer molecularly oriented film, said processcomprising the steps of:(a) forming a multiple layer film having firstand second layers whose compositions comprise at least 10% by weightethylene vinyl acetate copolymer and a third layer of vinylidenechloride copolymer disposed between said first and second layers; (b)heating said multiple layer film to an elevated temperature appropriatefor molecular orientation, and molecularly orienting said multiple layerfilm; and (c) after said orienting of said multiple layer film,subjecting said multiple layer film, containing said first, second, andthird layers, to irradiation in an amount of at least 1.5 megarads, saidirradiation after said orienting comprising an initial irradiationexposure for each of said first, second, and third layers.
 2. A processas in claim 1 and wherein the composition of at least one of said firstand second layers is a blend of 10% by weight to 90% by weight linearlow density polyethylene and 90% to 10% ethylene vinyl acetate.
 3. Aprocess as in claim 2 wherein said multiple layer film is formed having,as the composition of said third layer, a vinylidenechloride-methylacrylate copolymer.
 4. A process for making a multiplelayer molecularly oriented film, said process comprising the stepsof:(a) forming a multiple layer film having first and second layerswhose compositions comprise at least 10% by weight ethylene vinylacetate copolymer, and a third layer of vinylidene chloride copolymer,disposed between said first and second layers; (b) heating said multiplelayer film to an elevated temperature appropriate for molecularorientation, and molecularly orienting said multiple layer film; (c)holding said film at an appropriate elevated temperature and therebyheat setting it; and (d) after said orienting of said multiple layerfilm, subjecting said multiple layer film, containing said first,second, and third layers, to irradiation in an amount of at least 1.5megarads, said irradiation after said orienting comprising an initialirradiation exposure for each of said first, second, and third layers.5. A process as in claim 4 wherein the composition of at least one ofsaid first and second layers is a blend of 10% by weight to 90% byweight linear low density polyethylene and 90% to 10% ethylene vinylacetate.
 6. A process as in claim 4 wherein said multiple layer film isformed having, as the composition of said third layer, a vinylidenechloride-methyacrylate copolymer.
 7. A process for making a multiplelayer molecularly oriented film, said process comprising the stepsof:(a) forming a multiple layer film having first and second layerswhose compositions comprise at least 10% by weight ethylene vinylacetate copolymer and a third layer of vinylidenechloride-methylacrylate copolymer disposed between said first and secondlayers: (b) heating said multiple layer film to an elevated temperatureappropriate for molecular orientation, and molecularly orienting saidmultiple layer film; and (c) subjecting said multiple layer film,containing said first, second, and third layers, to irradiation in anamount of at least 1.5 megarads, and wherein said heating and orientingin subparagraph (b) is performed before irradiation of any one of saidfirst, second, and third layers.
 8. A process for making a multiplelayer molecularly oriented film, said process comprising the stepsof:(a) forming a multiple layer film having first and second layerswhose compositions comprise at least 10% by weight ethylene vinylacetate copolymer and a third layer of vinylidenechloride-methylacrylate copolymer, disposed between said first andsecond layers; (b) heating said multiple layer film to an elevatedtemperature appropriate for molecular orientation, and molecularlyorienting said multiple layer film; (c) holding said film at anappropriate elevated temperature and thereby heat setting said multiplelayer film; and (d) subjecting said multiple layer film, containing saidfirst, second, and third layers, to irradiation in an amount of at least1.5 megarads, and wherein said heating and orienting of subparagraph (b)is performed before irradiation of any one of said first, second, andthird layers.
 9. A process for making a multiple layer film, saidprocess comprising the steps of:(a) forming a multiple layer film having(i) first and second layers comprising about 65% to 75% by weightethylene vinyl acetate, said ethylene vinyl acetate comprising 7 to 9mole percent vinyl acetate, and 35% to 25% by weight linear low densitypolyethylene copolymer and (ii) a third layer of vinylidene chloridemethylacrylate copolymer disposed between said first and second layers;(b) subjecting said multiple layer film, containing said first, secondand third layers to irradiation in an amount of at least 1.5 megarads;and (c) including, before said irradiation of any of said first, second,and third layers, the step of heating said multiple layer film to anelevated temperature appropriate for molecular orientation andmolecularly orienting it.
 10. A process as in claim 9 and including thestep of holding said film at an appropriate elevated temperature andthereby heat setting it.
 11. A process for making a multiple layermolecularly oriented film, said process comprising the steps of:(a)coextruding a multiple layer film having first and second layers whosecompositions comprise at least 10% by weight ethylene vinyl acetatecopolymer and a third layer of vinylidene chloride-methylacrylatecopolymer disposed between said first and second layers; (b) beforeirradiation of said coextruded multiple layer film, heating saidmultiple layer film to an elevated temperature appropriate for molecularorientation, and molecularly orienting said multiple layer film; and (c)after said orienting of said multiple layer film, subjecting saidmultiple layer film, comprising said first, second, and third layers, toirradiation in an amount of at least 1.5 megarads, said irradiationafter said orienting comprising an initial irradiation exposure for eachof said first, second, and third layers.
 12. A process as in claim 11wherein the composition of at least one of said first and second layersis a blend comprising 10% by weight to 90% by weight linear low densitypolyethylene and 90% to 10% ethylene vinyl acetate.
 13. A process formaking a multiple layer molecularly oriented film, said processcomprising the steps of:(a) coextruding a multiple layer film havingfirst and second layers whose compositions comprise at least 10% byweight ethylene vinyl acetate copolymer and a third layer of vinylidenechloride-methylacrylate copolymer disposed between said first and secondlayers; (b) before irradiation of said coextruded multiple layer film,heating said multiple layer film to an elevated temperature appropriatefor molecular orientation, and molecularly orienting said multiple layerfilm; (c) holding said film at an appropriate elevated temperature andthereby heat setting it; and (d) after said orienting of said multiplelayer film, subjecting said multiple layer film, comprising said first,second, and third layers, to electron beam irradiation in an amount ofat least 1.5 megarads, said irradiation after said orienting comprisingan initial irradiation exposure for each of said first, second, andthird layers.
 14. A process as in claim 13 wherein the composition of atleast one of said first and second layers is a blend comprising 10% byweight to 90% by weight linear low density polyethylene and 90% to 10%ethylene vinyl acetate.
 15. A process for making a multiple layermolecularly oriented film, said process comprising the steps of:(a)coextruding a multiple layer film having first and second layers whosecompositions comprise at least 10% by weight ethylene vinyl acetatecopolymer and a third layer comprising vinylidenechloride-methylacrylate copolymer disposed between said first and secondlayers; (b) heating said multiple layer film to an elevated temperatureappropriate for molecular orientation, and molecularly orienting saidmultiple layer film; and (c) subjecting said multiple layer film,comprising said first, second, and third layers to irradiation in anamount of at least 1.5 megarads, and, wherein said heating and orientingin subparagraph (b) is performed before irradiation of any layer of saidmultiple layer film.
 16. A process as in claim 15 wherein thecomposition of at least one of said first and second layers is a blendcomprising 10% by weight to 90% by weight linear low densitypolyethylene and 90% to 10% ethylene vinyl acetate.
 17. A process formaking a multiple layer film, said process comprising the steps of:(a)coextruding a multiple layer film having (i) first and second layerscomprising about 65% to 75% by weight ethylene vinyl acetate, saidethylene vinyl acetate comprising 7 to 9 mole percent vinyl acetate, and35% to 25% linear low density polyethylene copolymer and (ii) a thirdlayer of vinylidene chloride-methylacrylate copolymer disposed betweensaid first and second layers; (b) subjecting said multiple layer film,containing said first, second, and third layers to irradiation in anamount of at least 1.5 megarads; and (c) including, before saidirradiation of said multiple layer film, the step of heating saidmultiple layer film to an elevated temperature appropriate for molecularorientation, and molecularly orienting it.
 18. A process as in claim 17,said irradiation after said orientation comprising an initialirradiation exposure for each of said first, second, and third layers.19. A process as in claim 11, said multiple layer film comprising aplurality of layer interfaces, and wherein said film cannot be peeledapart at any one of said interfaces.
 20. A process as in claim 12, saidmultiple layer film comprising a plurality of layer interfaces, andwherein said film cannot be peeled apart at any one of said interfaces.21. A process as in claim 13, said multiple layer film comprising aplurality of layer interfaces, and wherein said film cannot be peeledapart at any one of said interfaces.
 22. A process as in claim 14, saidmultiple layer film comprising a plurality of layer interfaces, andwherein said film cannot be peeled apart at any one of said interfaces.23. A process as in claim 15, said multiple layer film comprising aplurality of layer interfaces, and wherein said film cannot be peeledapart at any one of said interfaces.
 24. A process as in claim 16, saidmultiple layer film comprising a plurality of layer interfaces, andwherein said film cannot be peeled apart at any one of said interfaces.25. A process as in claim 18, said multiple layer film comprising aplurality of layer interfaces, and wherein said film cannot be peeledapart at any one of said interfaces.